[whylo]

I'm more of a nut ;) but I think anyone selling a service like this, or building any kind of front-end framework, has even more of a responsibility than individual site owners to make their stuff accessible, because any issues are multiplied out across all of your customers, and ultimately affect any of their customers with accessibility needs.

Depending on the juristiction your customers operate in, any accessibility issues could also expose them to legal liability for non-compliance with various laws (eg ADA in the US, the Web Accessibility Directive in the EU for public sector bodies)